As air pollution is becoming an increasingly serious problem, air purifiers have been widely used to improve air quality at living places. The purifying effect of an air purifier is generally determined by a filter, which is a core element of the air purifier. With the accumulation of usage duration, the filter is depleted gradually. However, due to lack of awareness about the status of the filter, users often fail to replace the filter timely. This may eventually make the air purifier fail to operate properly, and thus unable to effectively purify the air. Therefore, to establish a better living environment, it is necessary to provide alert about the current status of the filter, so as to keep the air purifier in a normal operating condition.